Both high strength (proof stress) and high corrosion resistance are required of steel materials, particularly steel pipes, for use in drilling oil wells, oil production, oil purification, oil transportation, and like applications; thus, various materials have been developed in accordance with working environment and applications. Particularly, for application to a wet carbon dioxide environment containing a trace quantity of hydrogen sulfide, dual-phase stainless steel having excellent corrosion resistance is often used. Dual-phase stainless steel has a microstructure of dual phase, i.e. a ferritic phase (hereinafter referred to as ferrite) and an austenitic phase (hereinafter referred as austenite). Since dual-phase stainless steel in the solid-solution heat-treated state is low in proof strength, proof strength is enhanced through cold working.
When dual-phase stainless steel is used as, for example, oil country tubular goods, dual-phase stainless steel pipes having a length of 10 m to 15 m are joined one to another through use of threaded joints as in the case of steel pipes of other materials. Since oil country tubular goods for use in drilling an oil well reach a final overall length of thousands of meters, hundreds of portions joined by means of threaded joints are involved. This method which uses threaded joints involves the following problems.
a) Since accurate threads must be cut, considerable cost is involved. PA1 b) Since a joining force varies among threaded joints, skilled workers must perform the joining work in order to ensure reliability of joined portions. PA1 c) Threads of pipes are susceptible to damage during transport. PA1 A) The ferritic phase percentage becomes relatively high over a weld zone and over a wide heat affected zone, causing impairment in toughness and corrosion resistance. PA1 B) Carbon-nitrides, intermetallic compounds, and the like precipitate over a wide heat affected zone, causing impairment in toughness. PA1 (a) Through the prevention of release of nitrogen (N) from a bonded portion and the vicinity thereof, there can be prevented an increase in the ferritic phase percentage thereof. The prevention of release of N can be implemented by adjustment of the atmosphere around the bond portion and the vicinity thereof. PA1 (b) In order to prevent an increase in the ferritic phase percentage in a bond portion and the vicinity thereof, in addition to the above-mentioned prevention of release of N, the upper limit of a rate of cooling from a diffusion-bonding temperature must be controlled to a predetermined value or lower. Also, by limitation of the lower limit of the cooling rate to a predetermined value or higher, the precipitation of intermetallic compounds, which would otherwise significantly impair toughness, is very effectively prevented. PA1 (c) Through the quantitative determination of the length of a heating zone which is not impaired in it's toughness or corrosion resistance during diffusion-bonding, and on the basis of the obtained quantitative data, the zone where carbon-nitrides and intermetallic compounds are generated is strictly limited. This limitation of a heating zone is also effective in obtaining a required proof stress to a joint. PA1 (d) In order to obtain a required proof stress to a joint, a material to be bonded is cold-worked at the vicinity of an end surface to be bonded or over the entire length so as to endow the end portion or the entire material with a proof stress which is at least a predetermined amount higher than the required proof stress of the joint. PA1 1) Butting portions and the vicinities thereof are heated to a temperature not lower than the melting point of the insert material and lower than the melting point of the dual-phase stainless steel and are held at the temperature for 120 seconds or longer, PA1 2) During heating as described above in 1), heating the zone not longer than 20 mm from the butting surface to a temperature not lower than 800.degree. C., and heating the zone not longer than 40 mm from the butting surface to a temperature not lower than 600.degree. C., PA1 3) After the heating and diffusion-bonding are performed, under the above conditions 1) and 2), cooling the bonded portion to a temperature of 400.degree. C., at a cooling rate of 50-150.degree. C./s.
To solve these problems, a welding method such as TIG welding is proposed as an alternative to the joining method which uses threaded joints. The welding method is a highly efficient joining method, but involves the following problems.
As a simple, convenient method to solve these problems involved in the welding method, a diffusion-bonding method is proposed (Japanese Patent Laid-Open (kokai) No. 3-86367). However, even the diffusion-bonding method has a drawback that softening occurs in a zone heated during diffusion-bonding, causing impairment in proof stress of a joint. Hereinafter, a joint refers to a wide zone which includes a bond portion, a heat affected zone, and a portion of base metal adjacent to the heat affected zone.
To solve the above-mentioned drawback, there is disclosed a method in which a zone including an end surface of base metal, which zone will become a joint, is previously cold-worked so as to improve the proof stress of the zone and in which a zone to be heated is narrowed so as to prevent impairment in strength (Japanese Patent Laid-Open (kokai) No. 6-7967). However, since the method described in Japanese Patent Laid-Open (kokai) No. 6-7967 is primarily concerned with a high-alloy steel pipe having an austenitic microstructure, an approach in relation to control of dual-phase microstructure composed of ferrite and austenite is not addressed. Accordingly, when the method of Japanese Patent Laid-Open (kokai) No. 6-7967 is applied to dual-phase stainless steel, the following problems arise.
1) Since the ferritic phase percentage of a bond portion becomes high as compared to that of base metal, the bond portion is impaired in corrosion resistance and toughness.
2) Since carbon-nitrides and intermetallic compounds are formed from ferrite in a bond portion, the bond portion is impaired in toughness and corrosion resistance.
So far, there has not been developed a method for bonding dual-phase stainless steel capable of simply and conveniently obtaining a joint having excellent strength and corrosion resistance.
An object of the present invention is to provide a simple, convenient method for bonding dual-phase stainless steel, particularly dual-phase stainless steel pipes, capable of providing a joint having excellent strength and corrosion resistance.